It is often desirable in continuously filling large numbers of containers in a commercial assembly line environment to check the weight of a charge of material that would be deposited in a container, in order to observe whether or not the filling device is actually placing the desired charge weight in the containers being filled.
In the past, this check-weighing has been accomplished in several different ways. One way is to take the gross weight minus the average tared weight. This method is used where the tared weight of each package is approximately the same and the net weight is usually considerably heavier than the tared weight. When these conditions exist, weighing the product and the container together is the simplest and most accurate method of check-weighing. However, as the net weight becomes close to the tared weight, the accuracies of this system begin to fail as discrepancies in tared weights will be reflected as discrepancies in the net weight.
A further way is to take the gross weight minus the specific tared weight. This method is used where the tared weight can vary considerably from container to container and the tared weight is close to or greater than the net weight. In this situation, a sample container is weighed before product is filled into the container and this specific container is again weighed after filling and the difference between the weights is said to be the net weight. This system has a distinct disadvantage, particularly when the tared weight is far in excess of the net weight. In this situation, small inaccuracies in the weighing of the container can be reflected as large variations in the net weight. This method also has the disadvantage that in production situations it is difficult to insert tared containers into and remove them from the production line after being filled without disrupting production. An example of this method of check-weighing is found in the pharmaceutical industry where as little as one gram of product may be filled into a container weighing 100 grams.
Yet another way is direct net weigh. In this method, the product is poured or knocked out of the container onto a pan of the scale or the filled container is weighed and the product is knocked out as waste and the empty container re-weighed. The net weight is then the difference between the two weighings. All of the above methods have the disadvantage of being time consuming, they usually result in the destruction of the container and are inaccurate to the extent that all of the material may not be dispensed from the container.